This application claims the priority of Japanese Patent Application No. 9-210113 filed on Jul. 17, 1997.
1. Field of the Invention
The present invention relates to a lithium secondary battery which comprises a positive electrode, a negative electrode comprising a lithium ion-occlusion material and a nonaqueous electrolyte and, more particularly to improvements in the lithium ion-occlusion material to be used in the negative electrode for the purpose of providing a lithium secondary battery having a large discharge capacity and good charge-discharge cycle characteristics.
2. Description of the Prior Art
Carbonaceous materials are well known as lithium ion-occlusion materials to be used in the negative electrode of lithium secondary batteries.
However, since carbonaceous materials have conductivity, overcharge may possibly result in ramiform deposition of metallic lithium on the surface thereof. Therefore, when a carbonaceous material is used, it is necessary to prevent overcharge of the carbonaceous material by decreasing the capacity of the positive electrode and/or using a charger provided with an overcharge preventing function, for instance.
Therefore, oxides of elements of the group IVB or VB of the periodic table, for example oxides of Ge, Sn and so on, have been proposed as lithium ion-occlusion materials for negative electrodes substituting for the carbonaceous materials (cf. Japanese Kokai Tokkyo Koho H07-122274). It is stated that by using these oxides, it is possible to obtain secondary batteries which have a relatively large discharge capacity and do not cause ramiform deposition of metallic lithium on the surface of the negative electrode even upon overcharge.
However, check experiments made by the present inventors revealed that when these oxides are used as lithium ion-occlusion materials for negative electrodes, the oxide structure rapidly undergoes destruction upon repeated charge and discharge, namely repeated lithium ion incorporation and elimination, whereby the discharge capacity decreases in a small number of charge-discharge cycles. Thus, it was revealed that the secondary batteries disclosed in Japanese Kokai Tokkyo Koho H07-122274 have a problem in terms of charge-discharge cycle characteristics.
Accordingly, it is an object of the present invention to provide a lithium secondary battery having a large discharge capacity and good charge-discharge cycle characteristics.